1. Technical Field
The invention concerns plasma reactors used in processing workpieces in the manufacturing of items such as microelectronic circuits, flat panel displays and the like, and in particular plasma sources therefor.
2. Background Art
The trend in microelectronic circuits toward ever increasing densities and smaller feature sizes continues to make plasma processing of such devices more difficult. For example, the diameter of contact holes has been reduced while the hole depth has increased. During plasma-enhanced etching of a dielectric film on a silicon wafer, for example, the etch selectivity of the dielectric material (e.g. silicon dioxide) to photoresist must be sufficient to allow the etch process to etch a contact hole whose diameter is ten to fifteen times its depth, without appreciably disturbing the photoresist mask defining the hole. This task is made even more difficult because the recent trend toward shorter wavelength light for finer photolithography requires a thinner photoresist layer, so that the dielectric-to-photoresist etch selectivity must be greater than ever. This requirement is more readily met using processes having relatively low etch rates, such as dielectric etch processes employing a capacitively coupled plasma. The plasma density of a capacitively coupled plasma is relatively less than that of an inductively coupled plasma, and the capacitively coupled plasma etch process exhibits good dielectric-to-photoresist etch selectivity. The problem with the capacitively coupled process is that it is slow and therefore relatively less productive. Another problem that arises in such etch processes is non-uniform plasma distribution.
In order to increase productivity or etch rate, higher density plasmas have been employed. Typically, the high density plasma is an inductively coupled plasma. However, the process precursor gases tend to dissociate more rapidly in such a high density plasma, creating a higher plasma content of free fluorine, a species which reduces the etch selectivity to photoresist. To reduce this tendency, fluorocarbon process gases such as C4F8 or C4F6 are employed which dissociate in a plasma into fluorine-containing etchant species and one or more polymer species which tend to accumulate on non-oxide containing surfaces such as photoresist. This tends to increase etch selectivity. The oxygen in the oxygen-containing dielectric material promotes the pyrolization of the polymer over the dielectric, so that the polymer is removed, allowing the dielectric material to be etched while the non-oxygen containing material (e.g., the photoresist) continues to be covered by the polymer and therefore protected from the etchant. The problem is that the increase in contact opening depth and decrease in photoresist thickness to accommodate more advanced device designs has rendered the high density plasma process more likely to harm the photoresist layer during dielectric etching. As the plasma density is increased to improve etch rate, a more polymer-rich plasma must be used to protect the non-oxygen containing material such as photoresist, so that the rate of polymer removal from the oxygen-containing dielectric surfaces slows appreciably, particularly in small confined regions such as the bottom of a narrow contact opening. The result is that, while the photoresist may be adequately protected, the possibility is increased for the etch process to be blocked by polymer accumulation once a contact opening reaches a certain depth. Typically, the etch stop depth is less than the required depth of the contact opening so that the device fails. The contact opening may provide connection between an upper polysilicon or metal conductor layer and a lower polysilicon conductor or metal layer through an intermediate insulating silicon dioxide layer. Device failure occurs, for example, where the etch stop depth is less than the distance between the upper and lower polysilicon or metal layers. Alternatively, the possibility arises of the process window for achieving a high density plasma without etch stop becoming too narrow for practical or reliable application to the more advanced device designs such as those having contact openings with aspect ratios of 10:1 or 15:1.
What would be desirable at present is a reactor that has the etch rate of an inductively coupled plasma reactor (having a high density plasma) with the selectivity of a capacitively coupled reactor. It has been difficult to realize the advantages of both types of reactors in a single reactor.
One problem with high density inductively coupled plasma reactors, particularly of the type having an overhead coil antenna facing the wafer or workpiece, is that as the power applied to the coil antenna is increased to enhance the etch rate, the wafer-to-ceiling gap must be sufficiently large so that the power is absorbed in the plasma region well above the wafer. This avoids a risk of device damage on the wafer due to strong RF fields. Moreover, for high levels of RF power applied to the overhead coil antenna, the wafer-to-ceiling gap must be relatively large, and therefore the benefits of a small gap cannot be realized.
If the ceiling is a semiconductive window for the RF field of an inductively coupled reactor, or a conductive electrode of a capacitively coupled reactor, then one benefit of a small wafer-to-ceiling gap is an enhanced electric potential or ground reference that the ceiling could provide across the plane of the wafer at a relatively small gap distance (e.g., on the order of 1 or 2 inches).
Therefore, it would be desirable to have a reactor not only having the selectivity of a capacitively coupled reactor with the ion density and etch rate of an inductively coupled reactor, but further having none of the conventional limitations on the wafer-to-ceiling gap length other than a fundamental limit, such as the plasma sheath thickness, for example. It would further be desirable to have a reactor having the selectivity of a capacitively coupled reactor and the etch rate of an inductively coupled reactor in which the ion density and etch rate can be enhanced without necessarily increasing the applied RF plasma source power.
The foregoing needs and desires have been met by the torroidal plasma source of the above-referenced parent application. The torroidal plasma source of the parent application is described below with reference to FIGS. 1-43. Very briefly, it involves a plasma reactor chamber with at least one external reentrant tube extending between ports on opposite sides of the chamber, and an RF power applicator such as an RF-driven magnetic core encircling a portion of the external tube. RF plasma current flow circulates through the tube and across the space between the ports. This space is the process region overlying the surface of a workpiece or semiconductor wafer to be processed, the workpiece surface being generally parallel to the RF plasma current flow.
One limitation of such a torroidal source is that the plasma ion density over the center of the workpiece or wafer tends to be less than the remaining regions. What is needed is some way of enhancing plasma ion density over the center of the wafer. One way to enhance it might be to use an overhead solenoid or coil that produces a cusp-shaped field confining plasma near the center. However, such a solenoid or coil necessarily has a significant radius relative to the small size of the center region having inferior plasma ion density. Therefore, such an overhead coil or solenoid tends to exert magnetic pressure on the plasma at a significant radial distance from the center and therefore tends to have less or little effect at the center itself, and thus may offer no improvement. Thus, what is needed is a way of enhancing plasma density over the wafer center that is effective within a small radial area around the center.
Another problem with the torroidal plasma source is that zones of high plasma ion density (“hot spots”) tend to appear near the wafer edge adjacent the center of each of the ports connected to the external tube. Accordingly, a way of eliminating such hot spots is needed that is non-invasive—that is, a way that does not create other non-uniformities in plasma ion distribution while eliminating the hot spots.
A further problem with the torroidal plasma source is that at least some of the recirculating plasma current induced in the reentrant tube can pass around the processing region by diversion through the pumping annulus below the plane of the wafer or workpiece. Such diversion reduces plasma ion density over the wafer, and therefore process performance can be greatly improved if diversion of plasma current through the pumping annulus can be prevented.
Still another problem is that the magnetic core encircling a portion of the external reentrant tube tends to crack or shatter when a relatively high level of RF power (e.g., several kiloWatts) is applied. This problem appears to arise from non-uniform heating of the core to due fringing magnetic flux. Therefore, a way of providing uniform distribution of magnetic flux in the core is needed.